Senior Mixed-Signal/Embedded Engineer — AC Power Safety Device (Impedance Measurement)

We are a US product company with a mature AC-line-powered device in field trials. The core of the remaining work is a precision small-signal impedance measurement made through a live AC power connection: it currently uses amplitude-only detection and needs to be rebuilt as synchronous (lock-in) detection with complex-impedance discrimination, plus a short list of firmware corrections that are already specified. You will not be guessing at requirements. A complete engineering review exists — root causes identified at file/line level, every change written as a formal spec with acceptance tests, and a defined bench validation matrix. Details are shared under NDA. You'll work alongside our staff engineer (who owns the design) and report directly to the owner. ## Scope (phased, milestone-based) - **Phase 0 — blind diagnostic pilot Step 1: you receive one schematic section and the observed field behavior — and nothing else. You deliver your own written diagnosis and proposed approach. Step 2: we share our engineering review; you deliver a short reconciliation — agree, disagree, and anything we missed. This is how we choose. Strong candidates tell us this part is fun. - **Phase 1:** implement a small set of blocking firmware corrections (fully specified, with acceptance tests) on a bare-metal Cortex-M C++ codebase. - **Phase 2:** implement the synchronous detection path — stable phase reference, complex-impedance decision logic, calibration, coherent averaging — and pass the bench validation matrix on hardware we ship you. - **Phase 3 (optional extension):** analog front-end evaluation and board-revision support. ## Required - Synchronous/lock-in detection or coherent demodulation implemented on an MCU (not just in a lab instrument) — you can explain phase-reference bookkeeping across acquisition windows - Strong bare-metal embedded C/C++: DMA-driven ADC/DAC pipelines, timer-triggered acquisition, Cortex-M - Analog measurement literacy: complex impedance, instrumentation amplifiers, coupling networks, noise/interference budgets - Bench skills: scope and signal source access; able to receive test hardware in the US - Clear written English; comfortable working against specs with acceptance tests ## Strongly preferred - AC line-powered or safety-critical product experience (functional-safety mindset; UL/IEC familiarity) - Impedance/instrumentation background (LCR, loop or insulation testing, power-quality, leakage protection — any of these) - Experience collaborating with (not replacing) an in-house engineer ## To apply, answer these briefly (we read every one) 1. Describe a lock-in/synchronous detection system you personally implemented on an MCU. How did you keep the demodulation reference phase-consistent across acquisition windows, and how did you prove stability? 2. You must measure a small impedance through a live AC power connection, and the reading can be fooled by parasitic paths. What parasitics would you expect, and how would you separate the real element from them? 3. Timer-triggered, DMA-sequenced ADC acquisition on Cortex-M: one concrete example you built. 4. Include the word "conductance" in your first line so we know you read this far. 5. Your rate, availability (hrs/week), and earliest start.